Formulae are given for thermodynamic isotope effects of simple fluids. These fluids are here treated as hard-sphere and square-well systems. A special method, the micropotential method, is used for the calculation oj classical thermodynamic functions. Quantum corrections to these functions are also given. Isotope effects due to the variation of the total mass (mass effects) are related to the estimation of quantum corrections of thermodynamic functions. Isotope effects due to the variation of potential parameters (potential effects) are calculated for methane. The potential effects of methnne are of the same order of magnitude as the corresponding mass effects.